1. Field of the Invention
This invention relates to a mold apparatus for use in the molding of parts of expanded synthetic resin, preferably having an open cell structure, typically parts of flexible polyurethane foam. It also relates to a method for molding such parts using the mold apparatus.
2. Prior Art
Automotive seat back pads are conventionally formed of flexible polyurethane foam. One exemplary pad is shown in FIG. 11. The pad 100 has a face portion 101, a side portion 102, and a back portion 103 which are integrally molded. The back portion 103 is formed with an opening 104 so that the back portion 103 is of an inverted U shape. Thus the pad 100 as a whole has a bag or curved channel shape. When it is desired to mold such a bag shaped pad 100, a mold assembly 51 as shown in FIGS. 12 and 13 is used in the prior art. More particularly, the mold assembly 51 is a three-sectional mold assembly consisting of a lower mold 52, an upper mold 53, and an insert 54. The lower mold 52 and the insert 54 define therebetween cavities 55a, 55b for forming the face and side portions. The upper mold 53 and insert 54 define therebetween cavities 56a, 56b for forming the back portion. A molded part is prepared by feeding a flexible polyurethane foam molding compound into the cavities 55a, 55b, 56a, 56b, fastening the molds, and causing the molding compound to expand.
Several problems arise in molding a pad as shown in FIG. 11. In order to mold an upper part 103a of the back portion 103 of the pad 100, generally known as a top pad part, the cavity 56a for forming the top pad part must be filled with the molding compound to its deepest point. To this end, it is a common practice to introduce the molding compound into the cavities 55a, 55b, 56a, 56b in slight excess of the necessity for thereby increasing the blowing pressure within the mold cavity. This results in the following problems. (1) An amount of molding compound in excess of the necessity is used. (2) As a result of extra feed of the molding compound in the cavity, more reaction gas is given off to increase the rate of rejection, particularly when a part of complex shape like the seat back pad shown in FIG. 11 is to be molded. (3) As the blowing pressure within the mold cavity increases, more burrs 106 are formed on the molded part 105 at the mold split positions or parting lines as shown in FIG. 14. After removal from the mold, an extra step is needed for deburring, tailoring and finishing.
In conjunction with the molding of automotive interior parts such as cushion pads from a polyurethane blow molding compound, a method for controlling the pressure in the mold cavity is proposed in Japanese U.M. Application Kokai No. 60418/1983. In this method, the mold cavity is defined between a lower mold and an insert. An upper mold is attached so as to cover the lower mold and the insert and define a pressure regulating chamber between the insert and the upper mold. The insert is provided with a vent hole for providing fluid communication between the chamber and the mold cavity. Then air in the cavity is discharged into the chamber through the vent hole.
Although this method uses the lower mold, insert and upper mold, the cavity for molding a part is defined by the lower mold and the insert. Therefore, the molded part is produced using only the lower mold and the insert while the upper mold only serves to define the pressure regulating chamber with the insert. In an essential sense, therefore, this mold assembly is a two-sectional mold rather than a three-sectional mold. Then this mold assembly cannot produce a bag-shaped part having a top pad portion 103a as shown in FIG. 11. It is still necessary to use a three-sectional mold as shown in FIGS. 12 and 13 when it is desired to mold a bag-shaped part as shown in FIG. 11. Also for the molding of undercut parts, the apparatus of Japanese U.M. Application Kokai No. 60418/1983 is not applicable.
The apparatus of Japanese U.M. Application Kokai No. 60418/1983 further suffers from an increased rate of rejection in molding pads of relatively simple shape because air voids are formed near the parting line. Since regulation of the pressure between the cavity and the chamber is performed by way of the vent hole, the vent hole must be increased in size or number. Then air is locally exhausted to disorder the expansion balance of foam. It is cumbersome to trim off burrs formed in the vent hole(s) at the end of molding.
Japanese Patent Publication (JP-B) No. 102583/1995 discloses a method for molding a polyurethane foam part comprising the steps of reducing the gas pressure in a mold cavity, introducing into the mold cavity a polyurethane stock in the form of a mixture of 100 parts by weight of a polyol component and 0.1 to 0.6 part by weight of water as a blowing agent, and expanding the stock to flow in and fill the cavity. In this method, however, the gas pressure in the mold cavity is reduced by evacuating the cavity through a gap at the parting plane between two mold sections. The periphery of the mold along the parting plane is surrounded by a closed chamber which is evacuated to exhaust air from the cavity through the gap between the parting planes. Burrs or fins are often formed in the gap between the parting planes.